1. Field of the Invention
This invention relates generally to motor oils and more particularly to low cost, low ash and low phosphorus lubricating oil formulations
2. Statement of the Prior Art
Sulfurized isobutylene and polyisobutylene such as di- and triisobutylene have long been known as additives for lubricating oils. They are in reality a complex mixture of products theorized to be principally 4,5-dialkyl-1,2-dithiole-3-thione and minor amounts of sulfides, polymeric sulfur-substituted compounds and mercaptans.
With the recent trend toward low-ash, low-phosphorus motor oils for improved catalyst efficiency, future motor oil formulations will be required which contain little or no phosphorus and the minimum amount of metallic additives. In Japan, this requires that the traditional amount of zinc dithiophosphate (ZDTP) in a motor oil be reduced to values of about 0.05%P and in the U.S. some car manufacturers will require phosphorous levels of 0.05% by 1985.
As reported by Torii, Kyozo et al "Anti-Wear Properties of Engine Oils-Effects of Oil Additives on Valve Train Wear" SAE Fuels & Lubes Meeting 6/7-9, 1977; this small amount of ZDTP appears to give acceptable wear control but places an additional demand on the lubricant in terms of oxidation control. SE quality motor oils cannot be economically developed using only the traditional phenolic or diphenylamine antioxidants in combination with the small amount of ZDTP, since even large amounts of the phenolic and diphenylamine antioxidants fail to give acceptable oxidation control in the Sequence IIIC Engine Test. Apparently, this is caused by an imbalance of the ZDTP/diphenylamine or phenolic synergism reported by Scott, Gerald "Atmospheric Oxidation and Anti-Oxidants" Elsevier Publishing Co., New York, 1965, at page 272.
Relevant patents in this field include U.S. Pat. Nos. 3,673,090, 2,995,569, 3,578,595, 3,897,353, 4,010,106, and 4,148,738. None of these hint or suggest in any manner applicants' unobvious combination of additives.